It is known that the use of this type of waveform necessitates making an allowance on the output power of the power amplifier (this allowance is likewise known by the expression Output Back-Off). The aim of this allowance is to remain within a region of linear operation of the power amplifier. However, the presence of this allowance is inconsistent with the quest for the best possible yield. The reason is that in order to improve the yield of the power transistors used in radio communications equipment, they are often focussed into class AB. One of the special features of the AB class is that its yield increases when the transmitted power increases. Another special feature of this class of operation is that its optimum operating point in terms of linearity is dependent on a certain number of operating variables such as the transmission frequency used or the temperature. These special features make it necessary to look for a compromise between yield and linearity for each waveform.
In order to solve this problem, devices that have an automatic power control loop (likewise known by the expressions ALC for Automatic Level Control) are known from the prior art.
Systems that use open-looped servo-control on the basis of a conversion table are known from the prior art. This table is produced in the factory, during production of the radio, and may possibly have an update mechanism. Thus, in these systems, the control of the transmission power will be dependent not on the signal at the output of the amplifier, but only on the signal at the input of the amplifier. These systems are very sensitive to the load variation of the antenna in a mobility situation and necessitate long calibration times, reducing the production capacities of the modules.
Systems operating in closed-looped mode are also known from the prior art. These systems are shown in FIG. 1, are connected to a modem 101 and have an amplification device 102 exhibiting a variable amplification gain. They also have a device 103 for determining a difference between the amplified signal and a copy of the signal to be amplified. Finally, these systems have a device 104 for determining the amplification gain on the basis of the difference.
The device 103 for determining a difference is known to carry out filtering of the amplified signal or of the signal representing the difference so as to remove the contribution of the variations in the modulation envelope on the gain control signal. The automatic gain control is then severely slowed down in relation to the spread band for the frequencies of the modulation used. In general, the loop band must be one hundred times lower than the bandwidth of the modulation in order to completely eliminate envelope variations. Thus, U.S. Pat. No. 7,023,278 B1 (Rockwell Collins, 2006) and U.S. Pat. No. 6,735,420 B2 (Globespan Virata, 2001) exhibit systems that use this solution. This type of system therefore cannot be used for amplifying signals that exhibit rapid variations in the modulation frequency (these signals are also known by the expression FH for Frequency Hopping) and that exhibit modulation in which the envelope is not constant. The reason is that these systems differentiate between an unmodulated setpoint signal and a modulated return signal, the effect of which is to create perturbations on the error signal that translate into a high level of imprecision on the variable gain control. During FH operation, this system is unacceptable because it does not have time to converge in a single transmission time interval on account of the need for the filtering to be very extensive.
The device 103 for determining a difference is known to make direct use of the samples of the signal to be amplified as a setpoint. It this case, the gain control loop can be rapid and it is possible to eliminate the envelope variations of the gain control signal. Thus, U.S. Pat. No. 7,353,006 B2 (Analog Devices, 2004) and U.S. Pat. No. 7,773,691 B2 (RF Micro devices, 2005) exhibit systems that use this solution. In these systems, it is possible to eliminate envelope variations subject to the group propagation time of the transmission chain not being too long, otherwise this likewise translates into a perturbation on the error signal and imprecision on the variable gain control.
The use is also known, to improve the performance of the automatic control, in which the device 104 for determining the amplification gain can take account of the perturbations of the signal that are generated by the amplification device 102. However, in the prior art systems, this taking-account of the perturbations is static, that is to say that it does not use an estimator to update the model of the perturbations of the amplification device. Thus, these systems can cause instability if the gain and the delay of the radio channel differ from the expected values.